The present invention relates to a cooling fan system for a vehicle with fuel cell propulsion, wherein air is moved by means of a cooling fan for cooling purposes through a heat exchanger and can thereafter be supplied to the environmental air either directly or indirectly after satisfying one or more further cooling tasks. The invention relates furthermore to a method for the operation of a fuel cell system.
Many proposals have already been made for the equipping of vehicles with fuel cell propulsion systems. Such vehicles are already being constructed and tested.
With such fuel cell propulsion systems, the fuel cells deliver electrical energy which, after appropriate processing, is applied to one or more drive motors which take care of the propulsion of the motor vehicle.
For fuel cell propulsion systems for vehicles, PEM (Proton Exchange Membrane) fuel cells are currently preferred which are connected in parallel and/or in series to one another and form a so-called stack. The fuel cells are supplied, on the one hand, with hydrogen from a suitable source. Protons which originate from the hydrogen and pass through the membranes of the fuel cells combine in the fuel cells with the oxygen of the air which is supplied to form water with the simultaneous generation of electrical energy.
The vehicle can be equipped with a hydrogen storage tank, and may, however, also be fed with a synthesized hydrogen-rich gas which is obtained from a hydrocarbon such as methanol. In this case, the hydrocarbon is processed in a processing device in the form of a so-called reformer to form the synthesized hydrogen-rich gas. When a reformer is used, it also requires air.
Fuel cells are also known which are directly fed with methanol, with the methanol consisting of up to 97% water. Such fuel cell systems require oxygen for the power generating reaction and must be supplied with air by a compressor.
Independently of the type of fuel cells which are used, a compressor is always required which makes available the compressed air for the fuel cells or for the reformer. Part of the output power of the fuel cell system is also applied to an electric motor which is required in operation to drive the compressor.
In practice, problems arise with the starting of a fuel cell system.
One known solution involves the use of a traction battery with, for example, 288 V operating potential. This traction battery has in principle three different tasks:
On the one hand, it is used to drive the main compressor in order to feed air compressed by this compressor into the fuel cell system, so that power is generated which then replaces the traction battery as the power source for the electric motor driving the compressor.
The second task of the traction battery is to assist dynamically the electric motor or electric motors which propel the vehicle so that, for example, with fast acceleration or at elevated speeds, the power of the traction battery supplements the electrical output power of the fuel cell system.
The third task lies in the fact that a traction battery can be used in order to realize regenerative braking for example. That is to say, on braking of the vehicle, the kinetic energy which is present is partly converted into electrical energy which can then be stored in the traction battery.
Although a traction battery can be useful for these different purposes, it represents an expensive and heavy component, so that one would be pleased to dispense with it. If, however, the traction battery is dispensed with, it could no longer be used to start up the fuel cell system.
For the starting of the fuel cell system, air is required. The air compressor is normally driven from the fuel cell potential; however, this is not yet available. In the absence of a traction battery, it has already been proposed to supply the fuel cell system with sufficient air by means of a 12 V auxiliary fan, i.e., a so-called start-up blower, so that the power generation starts there and the system can be gradually run up until the power generation by the fuel cell system is sufficient in order to maintain the system in operation.
Irrespective of whether one operates with a traction battery or with an auxiliary blower, many components which make the system more complicated or more expensive, such as fans, radiators, pipes, 288 V batteries, etc., are necessary which one would prefer to do without.
The object of the present invention is to make sufficient air available so that the fuel cell system can be started and run up without using a traction battery or an auxiliary fan.
In order to satisfy this object, provision is made, in accordance with the invention, for an air branching device to be provided which supplies at least a part of the air delivered by each fan to a duct and thereby enables the use of the branched-off air for the starting of the fuel cells and/or for the maintenance of the operation of the fuel cells.
In other words, it has been recognized, in accordance with the invention, when using a fuel cell propulsion system in which a cooling fan driven by the normal onboard battery is used for the cooling of liquid flowing through a heat exchanger, with the throughflowing air also being capable of being used for further cooling tasks, that the system consisting of the cooling fan and heat exchanger is unnecessary per se during the starting of the fuel cell system, since the heat which then arises is restricted, and that the cooling fan associated with the heat exchanger can thus be exploited for the starting of the fuel cell system until the fuel cell system delivers sufficient power to drive the compressor itself. That is to say, the cooling fan is used in the short term for the starting of the fuel cell system.
This can in principle take place in such a way that an air branching device, which is simply formed by a fixed guide wall, is provided and always branches off a part of the air flow produced by the cooling fan and supplies it via a suitable line to the fuel cells and/or to a reformer. On operation of the system, a device must then be provided in order to close off the duct when the compressor starts to run in order to prevent the compressed air generated by the compressor from escaping through the corresponding duct in the form of a reverse flow. This need could indeed be avoided if the branched-off air flow generated by the cooling fan is supplied to the inlet of the compressor and thus via the compressor to the fuel cells and/or the reformer.
A further possibility is to make the air branching device movable so that it can be switched between a first substantially inactive position and a second position which brings about the branching off of the air. In this manner it is not only possible to use part of the air flow delivered by the cooling fan, but rather the entire air flow can be used for the starting of the fuel cell system.
The cooling fan can in principle be a suction fan which is arranged downstream of the heat exchanger and sucks the air through the latter, with the air branching device then being arranged downstream of the suction fan. Such suction fans are customary with fuel cell propulsion systems.
It is, however, more favorable if each cooling fan is a pusher fan which is arranged in front of the heat exchanger. A fan of this kind is better able to deliver the airflow required for the starting of the fuel cells because it operates more efficiently under backpressure conditions.
When using a pusher fan, it is preferably connected to the heat exchanger by means of a housing which avoids air losses.
It is particularly favorable when an air guiding housing is arranged downstream of the heat exchanger directly adjacent to the latter so that all the air flowing through the heat exchanger enters into the air guiding housing.
Various possibilities exist in accordance with the invention in order to realize the air branching device. This can, in particular, be realized by adjustable plates, which, in a first position, permit the air moving through the heat exchanger to pass and, in a second position, close against one another in order to supply the air to the duct leading to the fuel cells. The plates are preferably arranged at the downstream side of the air guiding housing.
They can, for example, be arranged in the manner of a louver window. Another possibility is to arrange them in the manner of an iris diaphragm, with the plates forming a central opening in the state closed against one another, which lies opposite to the entry to the duct leading to the fuel cells.
The possibility also exists of arranging the plates in the manner of a roller shutter.
It is particularly favorable when an air collecting box extends over a region of the air guiding housing which is not covered over by the air branching device in the air branching-off position and collects the air deflected by the air branching device and directs it to the duct leading to the fuel cells. An air collecting box of this kind can be straightforwardly used with an air branching device consisting of plates, in particular if these are arranged in the manner of a louver window (Venetian blind) or in the manner of a roller shutter, since the air collecting box extends over the entire width of the heat exchanger and the plates can close against an edge region of the air collecting box.
A further possibility for realizing the air branching device and to use it with an air collecting box is to form the air branching device as a roller blind.
Instead of using an air collecting box, the air branching device can in the second position fully close off the air outlet side of the air guiding housing and the latter can have a connection for a duct leading to the fuel cells. In this way, the need to use a separate air collecting box is avoided, i.e., the air guiding housing, which is in any event present, is itself used as the air collecting box.
With embodiments utilizing an adjustable air branching device, the positioning motor for the positioning of the same is preferably mounted on the air guiding housing.
An air filter can be built into the air collecting box or into the duct leading to the fuel cells in order to ensure that the fuel cells and/or the reformer are only fed with clean air.
It is particularly favorable that in the invention the cooling system consisting of the cooling fan and the heat exchanger with the air branching device and the housings associated therewith, as well as any motor for the positioning of the air branching device, can be built up as a module, since it can then be designed in a space-saving manner as a unit which is easily exchangeable and which can be manufactured at favorable cost with a minimum of additional parts.
Finally, there is provided, in accordance with the invention, a method for the operation of a fuel cell system comprising a heat exchanger with at least one fan which produces a cooling air flow through the heat exchanger and with the a fuel cell arrangement fed with compressed air from a compressor, characterized in that at least a part of the cooling air flow is supplied to the fuel cell arrangement, and optionally to a reforming device connected in front of the fuel cell arrangement, for the starting of the fuel cell system and/or for the maintenance of operation in low load ranges, for example, during idling, during loss of speed by rolling or during overrun operation.